1. Field of Invention
The invention relates to methods for the surface modification of hydrophilic nano-talc slurries and the formation of hydrophobic nano-talc powders for polymer composites.
2. Description of Related Art
Talc is a naturally occurring mineral, a layered hydrous magnesium silicate of general empirical formula Mg3Si4O10(OH)2, that is broken up and usually ground to a fine powder. Talc is a white, apple green, gray powder with luster pearly or greasy with a Mohs hardness of 1-1.5. It has a high resistance to acids, alkalies and heat. The hydroxy groups normally are internal to the magnesium layer and are not accessible to water except at the edges of the silicate sheet. Thus, conventional talc powder is a hydrophobic material that easily blends and disperses with organic media including polymers but is not easily dispersed in aqueous media.
Earlier U.S. patent applications Ser. No. 10/890,852, filed Jul. 14, 2004 and 10/175,976, filed Jun. 20, 2002, commonly owned by same assignee as the application described herein, describe methods of providing an aqueous nano-talc slurry with a specific surface area of 70 m2/g to about 500 m2/g. The nano-talc powders provided by these methods show unusual properties for talc, in that the powders exhibit a hydrophilic rather than hydrophobic character. For instance, one such hydrophilic nano-talc composition, derived from the so-called hybrid milling process, is characterized by absorbing about 5 to about 15 wt % water at about 40% to about 60% relative humidity. Slurries of the same hydrophilic nano-talc remain suspended in water for months with very little sedimentation. Although these properties are of interest for aqueous based composite systems, the hydrophilic properties of the nano-talc powders is a disadvantage, when attempting to blend the nano-talc in organic polymers or other hydrophobic media. The inventors have found that surface modification of the nano-talc slurries with amphophilic cations effectively changes the nano-talc properties from hydrophilic to a hydrophobic character. Thus, the invention provides a hydrophobic nano-talc composition that is useful in preparing hydrophobic polymer composites.